Written by Mick Jagger in July and August of 1969, the notes were sent to Marsha Hunt, the American singer with whom the legendary frontman had his first child. Hunt is said to have been the inspiration for the Stones' "Brown Sugar," which you can hear below in a live rendition from a concert in Texas in 1972.

According to a report in the Guardian, Jagger describes his admiration for Emily Dickinson's poetry and a meeting with the writer Christopher Isherwood, whom Jagger evidently hoped would help cast him as Caligula in a film version of "I, Claudius."

"The letters speak for Mick at an incredible juncture of our lives," Hunt said. "The summer of '69 was the end of a whole era of revolutionary spirit -- we didn't know it was about to die. And who knew that this group of boys making music would 50 years on be still celebrated as a voice of the period?"

Sotheby's estimates that the letters -- there are 10 in all -- will fetch between £70,000 (about $111,300) and £100,000 (about $159,000), a sum Hunt said she'll use to repair her house in France.

"I'm broke," she told the Guardian. "Anyone who has the impression that I have money knows nothing about me."